The present invention relates to a coating type magnetic disk for use in memory apparatus, etc., and a process and an apparatus for producing the magnetic disk.
Heretofore, a coating type magnetic disk has been produced by dispersing magnetic powders in a binder (macromolecular binder) or its solution in a sand mill, kneader or ball mill, thereby preparing a magnetic paint and applying the magnetic paint to a substrate, followed by orientation, if required, and by heat curing and polishing. If further required, a lubricant is applied to the coating film. A thermo-setting resin is usually used as a binder, and thus can be thermally cured, and sometimes a binder may be used only with heating. The magnetic paint is stirred during the preservation to prevent the magnetic powders from reaggregation.
With recent progress of higher density magnetic disks, magnetic powders composed of finer particles have been used. In a magnetic paint containing such magnetic powders, the magnetic powders are liable to reaggregate and it is difficult to completely prevent the reaggregation only by the stirring. To solve this problem, Japanese Patent Application Kokai (Laid-open) No. 62-18629 proposes to uniformly apply a magnetic paint to a substrate to a predetermined thickness, applying an ultrasonic vibration thereto, and immediately thereafter applying a uniform magnetic field thereto, thereby orienting magnetic powders. Even if reaggregation takes place, the structure of reaggregation is broken by the proposed process and the rotational motion of magnetic powders by orientation under a magnetic field is facilitated to attain a high orientation and improve the hysteresis loop squareness. Japanese Patent Application Kokai (Laid-open) No. 60-83224 proposes to produce a perpendicularly oriented magnetic disk by applying a perpendicular magnetic field and ultrasonic vibration to the substrate.
However, in the above-mentioned Japanese Patent Application Kokai (Laid-open) No. 62-18629 no consideration is given to the production of a magnetic disk of higher density having distinguished electromagnetic properties as a longitudinally oriented magnetic recording medium, for example, good S/N (signal/noise) ratio and hysteresis loop squareness. That is, the above-mentioned process is directed to the production of not a magnetic disk, but a magnetic tape. A magnetic tape has a thick coating film (thickness: more than 1 .mu.m) and a magnetic paint as a material for such a coating film has a high viscosity, for example, 1,000 to 2,000 cp. Thus, in the magnetic paint once highly dispersed by ultrasonic vibration, the magnetic powders do not reaggregate in a short time and thus can be readily oriented by applying a magnetic field thereto right after the dispersion of magnetic powders in a coating film state. On the other hand, a magnetic disk of higher density has a thin coating film (thickness: not more than 0.75 .mu.m). As a material for such a coating film, a magnetic paint having a low viscosity, for example, not more than 500 cp, must be used. In a magnetic paint of low viscosity the magnetic powders more readily aggregate than in a magnetic paint of high viscosity, and thus the degree of aggregation in a coating film state is considerably higher in a magnetic paint of low viscosity than in that of high viscosity. Thus, the structure of aggregates is hard to break even by applying ultrasonic vibration, usually used in the above-mentioned prior art, thereto and the coating film is not brought into a highly dispersed state of magnetic powders. Furthermore, if there is a time difference, even though short, between the application of ultrasonic vibration and the application of magnetic field for orientation, the dispersed magnetic powders will aggregate again, corresponding to the time difference. The magnetic disks thus prepared have problems in electromagnetic properties, for example, deterioration of hysteresis loop squareness, etc.
The perpendicularly oriented magnetic disk disclosed in the above-mentioned Japanese Patent Application Kokai (Laid-open) No. 60-83224 has problems in electromagnetic properties such as a low S/N ratio, easy occurrence of defects, a low output, etc., as compared with the longitudinally oriented magnetic disk, owing to such a peculiar property as perpendicular orientation of magnetic powders.